Normal sinus rhythm of the heart begins with the sinoatrial node (or "SA node" ) generating an electrical impulse. The impulse usually propagates uniformly across the right and left atria and the atrial septum to the atrioventricular (AV) groove. This propagation causes the atria to contract.
At the atrioventricular (AV) groove, the impulse encounters the so-called "skeleton" of the heart. Here, a fibrous structure separates the atria from the ventricles. The rings or annuli of the tricuspid valve (between the right atrium and right ventricle) and the mitral (or bicuspid) valve (between the left atrium and the left ventricle) are attached to this fibrous skeleton.
The fibrous skeleton is electrically inert. It normally acts as an insulator to block the conduction of the impulse from the SA node. The electrical impulse would be prevented from crossing over to the ventricular side of the AV groove, if not for the specialized AV conducting tissue, called the atrioventricular node (or "AV node") and the bundle of HIS (or "HIS bundle").
The AV node slows the conduction of the impulse to the ventricles, allowing the atria to first complete their contraction and empties blood from the atria into the ventricles. The slowed impulse eventually enters the HIS bundle, which delivers the impulse to the ventricular side. The ventricles then contract.
The AV conduction system results in the described, organized sequence of myocardial contraction.
Normally, the AV conduction system is the only way for electrical impulses to be conducted from the atria to the ventricles. However, some people are born with additional electrical conduction paths between the atria and ventricles. These extra connections are called "bypass tracts" or "accessory pathways". Accessory pathways consist of tiny bands of myocardial tissue that most commonly insert in atrial muscle on one end and ventricular muscle on the other end. The most common variety is located along the AV groove.
Accessory pathways offer a potential parallel route for electrical impulses, bypassing the normal AV conduction system.
The accessory pathways do not slow down the electrical impulse, like the AV node does. Instead, the accessory pathways conduct impulses more quickly, like myocardial tissue. When they conduct the impulses in the antegrade direction (i.e., from the atria to the ventricles), they precede the normal impulse from AV node, causing premature stimulation and contraction of the ventricles. When they conduct the impulses in the retrograde direction (i.e., from the ventricles to the atria), the atria contract after the ventricles do. In either case, normal heart rhythm becomes disrupted.
Patients with accessory pathways are susceptible to re-entrant tachycardias involving both the AV node and the accessory pathway. The resultant fast heart rate can be potentially life-threatening. The elevated heart rate can lead to serious hemodynamic compromise. Sudden syncope or hemodynamic collapse can occur.
Accessory pathways are generally invisible to the naked eye. They therefore must be located by their electrophysiologic effects.
Catheter-based techniques have been developed to record accessory pathway activation by mapping the left and right annuli of, respectively, the tricuspid valve and the mitral valve. The techniques map these regions within the heart using sensing electrodes carried by catheters introduced by vascular access into the heart.
These catheter-based techniques have allowed identification of the site of the accessory pathway. Once identified, the pathway can be rendered non-conductive by catheter-based ablation techniques.
However, physicians frequently find it difficult to map and ablate around these annuli, particularly when working on the atrial side. Many complex movements are presently required to map the entire annulus using conventional catheters. Stable and intimate contact between the myocardial tissue and the mapping electrodes are often difficult to achieve.
Furthermore, conventional systems require the use of separate ablating elements. Coordinating the position of the mapping electrodes and the ablating electrodes further compounds the difficulties.
As a result, mapping and ablating of accessory pathways using conventional catheter-based techniques are difficult and time consuming. For these reasons, many attempts at creating curative lesions ultimately fail.
There is a need for catheter-based systems and methods that simplify the procedures for mapping and ablating accessory pathways.